1. Field of the Invention
This invention relates to apparatus for the continuous hot dip coating of metal sheet in strip form and more particularly to an improved bearing support system for the rolls submerged in the molten bath of a hot dip continuous metal strip coating apparatus.
2. Description of the Prior Art
A typical hot dip metal coating apparatus used in zinc and zinc alloy coating of continuous steel strip is schematically shown in U.S. Pat. No. 3,499,418 which illustrates a steel strip being prepared for coating in a cleaning and annealing furnace from which it passes through a controlled atmosphere directly into a molten zinc or zinc alloy bath. The metal strip extends downwardly into the molten metal and around a first submerged roll, referred to as a sink roll, then upwardly in contact with one or more submerged stabilizing rolls before exiting the bath. From the bath, the strip passes between a pair of opposed air knives which control the thickness of the metal coating, then upwardly through a cooling zone and over a top guide roll. Since hot dip zinc and zinc alloy coating (herein generally referred to as galvanizing) is a common form of molten metal coating operation, the invention will be described with reference to a galvanizing operation, it being understood that the invention may also be employed in other hot dip coating applications.
In galvanizing apparatus of the type just described, the sink roll and stabilizing rolls normally are supported on arms which project into the bath along the inner surface of the sidewalls of the molten spelter container (zinc pot) into the bath. The rolls are supported by bearing assemblies each preferably including a temperature and wear resistant sleeve or journal mounted on the projecting end of the roll shaft and an oversized bearing element or bushing mounted on the end of the support arm. Molten metal flowing into and through the oversized bushing acts as the only lubricant for the bearing.
The temperature of the molten zinc or zinc alloy coating metal may be in the range of about 950.degree. F. This high temperature, in combination with the high tensile loads required to be maintained in the strip to control its high speed movement through the apparatus, results in the roll bearing assemblies wearing quickly. With increased bearing wear, the molten zinc becomes less effective as a lubricant, thereby increasing friction which in turn accelerates wear on the bearing elements.
The combination of the oversized bushing and the directional load applied to a roll by the moving strip can result in roll movement, or bearing chatter, which is aggravated by bearing wear. Chatter or movement of the sink roll and guide rolls can produce strip movement at the air knives and set up vibrations in the strip between the guide rolls and the top roll. Excessive movement of the strip adversely affects uniformity of the coating thickness, and high frequency vibration can result in spatter of the molten coating metal and produce undesired irregularities or markings on the finished coating surface. These irregularities may adversely affect further finishing operations such as painting. Air knife operation can also be adversely affected by spatter which can result in molten metal contacting and being deposited on the knives and can require shut down of the operation for knife cleaning.
U.S. Pat. No. 5,099,780 discloses an improved bearing structure for use with submerged rolls in a hot dip galvanizing operation to substantially reduce bearing chatter and wear during the galvanizing operation. According to this prior patent, the journal bushing is provided with a pair of flat surfaces disposed in planes intersecting one another at an angle of about 60.degree. to about 135.degree. and contacting the roll journal substantially tangentially. The line of intersection of the plane bearing surfaces and the line of action of the force applied to the submerged roll by the strip being coated preferably lie in a common plane containing the roll axis so that wear on the two surfaces is substantially equal. While the rate of wear was reduced by the flat bearing surfaces, and by the more efficient liquid zinc lubrication resulting from the improved pumping action of the bearing design of this prior art patent, bearing wear and the ultimate onset of chatter remained a problem and replacement and/or reworking of the bearings was necessary at more frequent intervals than desired. It is, therefore, a primary object of the present invention to provide an improved roll support and bearing assembly for the submerged rolls used in a hot dip galvanizing line.
Another object is to provide such an improved bearing assembly which provides a greatly extended useful life.
Another object is to provide such an improved bearing assembly employing wear resistant inserts having substantially planar surfaces disposed at an angle with respect to one another to contact the roll journal along lines substantially equally spaced from the line of action of the resultant force from the tensile load applied by the moving strip being coated.
Another object is to provide such an improved bearing assembly in which wear resistant inserts are supported for cooperation with locating grooves carried by the roll support arms to orient the bearing in the desired position.